Optical fiber connectors and splices are an essential part of optical fiber communications systems. Connectors may be used to join lengths of optical fiber into longer lengths, or to connect optical fiber to active devices such as radiation sources, detectors, or repeaters, or to passive devices such as switches or attenuators.
An optical fiber connector must meet at least two requirements. It must couple or join two optical fibers with minimum insertion loss. Secondly, it must provide mechanical stability and protection to the junction between the optical fibers in the working environment. Achieving low insertion loss in coupling two optical fibers in generally a function of the alignment of the optical fiber ends, the width of the gap between the ends, and the optical surface condition of the ends. Stability and junction protection is generally a function of connector design, such as, for example, the minimization of differential thermal expansion effects.
Many approaches to achieving fiber alignment can be found in the prior art. Among them are V-grooves, resilient ferrules, and conical bushings. A discussion of prior art connectors is provided in R. Schultz, Proceedings of the Optical Fiber Conference, Los Angeles (September 1982), pp. 165-170.
Some prior art optical fiber connectors contain one or more precision-machined parts and therefore are relatively costly items. Whereas this may be acceptable for some applications, in other cases the cost of such prior art connectors might constitute a significant fraction of the total installation cost. Thus, strong incentives exist for providing optical fiber connectors that do not require expensive precision-machined parts.
A further consideration in connector design is the relative ease of field installation of the connector. It is desirable that a sought-after connector be capable of being installed within a relatively short period of time without requiring special skills or manipulations not easily carried out in the field. Further, it is desired that an optical fiber connector be capable of field-terminating a length of optical fiber.
A prior art connector which has many of the above-listed desirable features includes two drawn glass cylindrical plugs or ferrules, with a fiber end portion inserted into a close-fitting passageway of each ferrule, and the connection between the two fiber ends made by inserting the ferrules in end-to-end fashion into an alignment sleeve that maintains the outer surfaces of the two ferrule in registry. This connector design relies on the capability of producing ferrules to very close tolerances by drawing them from a glass preform. Relative rotation of the two ferrules typically changes the relative position of the fibers held within the passageway because of the eccentricity of the optical fiber core with respect to the ferrule. Eccentricity is defined as the distance between the longitudinal centroidal axis of the ferrule at an end face of the ferrule and the centroidal axis of the optical fiber core held within the passageway of the ferrule. Generally, the passageway is not concentric with the outer cylindrical surface which is the reference surface. Also, the optical fiber may not be centered within the ferrule passageway and the fiber core may not be concentric with the outer surface of the fiber. Hence, the eccentricity is comprised of the eccentricity of the optical fiber within the ferrule passageway and the eccentricity of the passageway within the ferrule.
Because it is very difficult to control the eccentricity of the optical fiber core in the ferrule in which it is terminated, it is difficult to achieve desired losses of 0.1 dB or less in single mode fibers without maintaining close tolerances so that the opposed cores are aligned to within about 0.7 .mu.m. This, of course, increases manufacturing costs.
If the total eccentricities of the two optical fiber ends to be joined are identical or at least very nearly so, then a low-loss connection can be achieved by merely rotating, within the alignment sleeve, one ferrule with respect to the other, until maximum coupling is observed.
Central to a so-called prealigned rotary splice is the recognition that eccentricity between ferrule passageway and ferrule cylindrical surfaces essentially will have no effect on alignment of fibers terminated by two ferrules if the two ferrules have essentially the same amount of passageway eccentricity relative to the cylindrical surfaces and if the ferrules are aligned such that the eccentricities are in the same radial direction from centroidal axes of the ferrules or are in the same quadrant.
Another popular optical fiber connector is one known as the SC connector. An SC connector includes a ferrule assembly which includes a barrel having a collar at one end and an optical fiber terminating ferrule projecting from the barrel. The ferrule assembly is disposed in a plug frame such that an end portion of the ferrule projects from one end of the plug frame and a strength member retention portion of a cable retention member is disposed over the barrel projecting from the other end. The plug frame is configured so that it is polarized with respect to a grip into which the plug frame snap-locks. One grip is inserted into one end of a coupler housing and another grip is inserted into another end of the coupler housing to cause the ends of the ferrules to become disposed in optical connection with each other.
The foregoing assembly is made so that the direction of eccentricity of the ferrule passageway becomes aligned with a key disposed on an outer surface of the grip. In order to cause the direction of eccentricity to become aligned with the key, inasmuch as the plug frame can only be inserted in one orientation, the ferrule must be oriented with respect to the plug frame prior to its assembly therewith. This is a somewhat difficult task and consumes excessive time. Also, whereas the SC connector has a number of advantageous features, in its present form it is difficult to use to field-terminate optical fibers.
What is sought after and what seemingly is not available in the art is an SC ferrule connector in which the eccentricity of the ferrule fiber-receiving passageway is aligned with a key of a grip and in which the eccentricity of the passageway may be determined after the ferrule assembly has been assembled with another portion of the connector and/or after the ferrule assembly has been used to terminate an end portion of an optical fiber. The capability of delayed eccentricity determination should reduce the cost of such a connector and render it more marketable to a wider segment of the industry. Also, sought after is an SC connector which is easily used for field termination of optical fiber.